Interferences: fundamentals
Coherence length
The coherence length of the source is defined by the relationship to the degree of coherence. We have:
The source's coherence time is thus given by:
where c is the velocity of the light in vacuum. The coherence time can be interpreted as the average temporal width of a wave front emitted by the source.
At an observation point M, if the optical path difference is greater than the coherence length, then the interference phenomenon can no longer be observed. The illumination appears uniformly white. This shade is called higher order white. When this illumination is analyzed, we can observe a discrete spectrum, which means that the illumination is missing some wavelengths; these correspond to destructive interferences.